1. Field of the Invention
The invention relates to a device with an oscillator circuit comprising a current source; a capacitive node; a switching circuit coupling the current source and the capacitive node, the switching circuit having a control input for switching the switching circuit at least into a first or a second state, wherein the capacitive node is charged from the current source or discharged, respectively; a loop circuit coupling the capacitive node to the control input, the loop circuit being capable of causing operation of the oscillator circuit in at least a first and a second mode, the first mode being an oscillating mode having an oscillating period affected by charging of the capacitive node substantially during a critical time-interval only; and a mode breaker circuit for providing a conduction path which frustrates charging of the capacitive node in the second mode.
2. Description of the Related Art
Such a device is known from International Patent Application WO 94/26026. This publication describes a ring-oscillator containing eight inverters in a loop.
This ring-oscillator has several modes of operation. In an oscillating mode (called the first mode), two mutually opposite signal transitions travel around the loop. When a falling signal transition reaches the input of an inverter, its output voltage will start to rise. When the output voltage has sufficiently risen, this will trigger the next inverter to start discharging, and so on. The period of oscillation is the sum of the critical time-intervals between the start of charging or discharging and the triggering of the next inverter.
In a static mode of operation (an example of what will be called the second mode), the inverters of the ring-oscillator carry time-independent values, half of the inverters carrying a logic "high", the other half carrying a logic "low".
The publication uses what is called mode breaker circuits which destabilize the static mode to ensure that the loop will operate in the oscillating mode. The inverters in the ring-oscillator are functionally organized into pairs of inverters that are separated from each other by half the loop. In the oscillating mode, the inverters in a pair will have logically opposite output signals, but in the static mode, the inverters in a pair have identical output signals, both low or both high. A mode breaker circuit is coupled between the outputs of the inverters of each pair. The mode breaker circuits force the output signals of each pair of inverters to become different in case they are both high, making it impossible for the loop to stay in the static mode indefinitely.
According to WO94/26026, each inverter contains a current source and a capacitive output node. A switching circuit, controlled by the preceding inverter in the loop, determines whether the output node is charged by the current source or discharged. The mode breaker circuit forces the output node of a first inverter of a pair to discharge if the output node of a second inverter of the pair is charged. Thus, in the static mode, the two output nodes cannot both remain charged at the same time.
In the oscillating mode, the inverters of a pair is in phase opposition, so that the current source of the first inverter has to start charging its output node when the second inverter starts discharging its output node. Once the output node of the second inverter has sufficiently discharged, the mode breaker circuit will not inhibit charging of the output node of the first inverter. When the current source has sufficiently charged the output node of the first inverter, the first inverter will be able to trigger discharging in the next inverter in the loop, ensuring oscillation in the oscillating mode.
In such an oscillator circuit, the oscillation turns out to suffer from jitter, i.e., noise in the phase of the oscillation.